a) Technical Field
The invention relates to an arrangement for plansifters including a plansifter sieve having a sieve box with base plate and fine-product discharge, an insert sieve frame and freely movable cleaning members.
A plansifter, particularly the large plansifter or square plansifter, is a large cube with large surface elements and is a special separating device typically employed in mill processing technique for sifting break, semolina, and flour products and for sorting different types of grain such as wheat, rye, corn, barley, etc.
b) Background Art
It is known in many separating techniques, e.g. in chemical processing technology, to reduce the product humidity, if necessary, so that the separating devices do not become clogged. In contrast, optimal grinding humidity must be achieved for processing ground grain products to form flours, semolina, middlings, etc. But since grinding and sifting must be performed repeatedly in a grain mill, the humidity is regulated according to the grinding and not to the best possible sifting results.
It is known that many grinding fractions have poor flow properties or sifting properties due to the high humidity as can also be experienced in the home kitchen when sifting flour.
In a plansifter, all possible sizes of sieve mesh are used--from the finest, e.g. 80 microns, to the coarsest. It is equally important in every case to achieve the maximum sieve output without clogging the sieve mesh. It must be possible to achieve maximum throughput with the highest possible quality of ground products. In mill sifting, as opposed to classical sieving techniques, a closed product layer amounting to several centimeters is maintained over the sieve covering. In this way, a greater downward pressure (force of gravity) against the sieve covering acts on the finer, heavier portions, resulting in a higher throughput and greater sieving rate. On the other hand, flatter husk parts should float to the top as coarse product with the layer as sieve tailings. This type of "thick-layer sifting" causes severe problems in the case of moist grinding products and has a strong tendency to clog the sieve screen.
Systematically over the last 100 years, higher-capacity plansifters have been developed and sieve cleaning has been steadily improved. Innumerable variations of shapes and materials have been tested and sometimes incorporated in practice. In large plansifters, few shapes have been retained. Interestingly, textile belt pieces of 5-10 centimeters in length with a metal knob in the center have been most widely used for cleaning the sieve screen. The sieve screen cleaners are inserted and locked in individually between the woven sieve covering and a supporting grate. For this reason, the underside of the sieve frame is covered with a metal grid as support for the sieve screen cleaners. The metal grid usually exhibits a slight waviness so that the metal knob is struck sharply by the metal grid. The belt piece moves about intensively in a jerky, noisy manner as a result of the oscillating or circular motion of the entire plansifter. In this way the mesh openings are kept free and clean by the fringed ends of the belt pieces. The metal grid and the metal knob serve an important function for the movement of the sieve screen cleaner. A disadvantage consists in that the movement also causes a very audible noise. Sieve cleaning pieces of plastic in various forms which work in the same manner are also known.
Another problem area in plansifters consists in keeping the base plate completely clear. The fine product (throughs) falling through the sieve mesh should be cleared off the base plate within the shortest possible period of time and removed through lateral slots. A separate category of cleaning members, so-called base cleaners, have also been developed for this purpose which conventionally have 3-cornered, 4-cornered, rhomboid or polygonal shapes.
Many factors influence the operation of the plansifters. Thus, a newly covered sieve frame generally works better, since the wear on the covering as well as the degree of tension of the sieve screen are important. However, experience shows that the capacity of a plansifter, even in the best condition, is subject to natural limits. The intensity of the oscillating movement, that is, the maximum acceleration, is also limited by the maximum allowable forces in the plansifter housing on the one hand and by the movement behavior of the product on the other hand. Excessive acceleration forces prevent the product from flowing through freely. Accordingly, the output data are determined depending on the specific quality of the individual sifting fractions.
It is suggested in DE-AS No. 25 06 981 to provide the base plate with a saw-toothed surface so that the product removal proceeds as quickly as possible due to corresponding impact forces. In very specific cases, it is entirely conceivable that this will bring about advantages. But, as far as is known, such a solution has not been introduced commercially to any extent. It would be expected that particularly difficult, fine grinding fractions would clog the base plates or sieve boxes very quickly. In a mill the plansifter is the machine requiring the greatest expenditure on cleaning. More recently it has become increasingly important for the processing machinery to be self-cleaning when possible and, when not, easy and inexpensive to clean.